This invention relates to a luminance signal/color signal separation circuit (hereinafter called a Yc separation circuit) for separating Yc (luminance) signal and Cc (carrier chrominance) signal from a composite video signal to take them out, e.g., in VTR, etc.
FIG. 1 shows a generally used C signal comb filter circuit. A video signal incoming to a terminal 1 is passed through a 1H delay circuit and a subtracter, and through a comb filter which passes all the frequency components of the video signal. The bandwidth of the video signal thus supplied is then limited at the following band-pass filter BPF and outputted as a chrominance signal.
It is preferable that the bandwidth of a comb filter used for Yc separation be wide for a C signal having a large level such as a color bar signal from the standpoint of resolution, and narrow for a C signal having a small level such as a signal for scenery from the standpoint of cross color (due to this, the image is unnecessarily colored). To match both the contradictory cases, the bandwidth of a comb filter has generally been set at about 500 KHz to 1 MHz in practice.
Generally, a Yc separation circuit cannot provide a perfect separation function so that when a signal for oblique lines as shown in FIG. 2 is incoming to the circuit shown in FIG. 1, a low level C signal is taken out therefrom instead of an essential Y signal. This low level C signal is called cross color. Such cross color cannot be fully eliminated although it depends on the bandwidth of the comb filter. And, there occurs a problem that the narrower the bandwidth of the comb filer is made, the more the filter characteristic is degraded.
FIG. 3 is a block diagram showing an example of a conventional Yc separation circuit. A composite video signal (e.g., color bar signal) incoming to a terminal 1 passes through a band-pass filter 2 and a filter circuit 3 described later and is taken out from a terminal 4 as a Cc signal. The composite video signal also passes through a .DELTA.t delay circuit 5 and a 1H delay circuit 6 and is added to the Cc signal at an adder 7. This Yc separation circuit utilizes a vertical correlation between video signals. A conventional comb filter has used a vertical correlation between two lines (present line information, and 1H past line information), whereas this circuit uses a vertical correlation among three lines (present line information, 1H past line information, and 1H future line information).
In the filter circuit 3, the three types of line information are represented by A, B and C, respectively, where A (past) is an input signal to the filter circuit 3, B (present) is an output from 1H delay circuit 9, and C (future) is an output from the 1H delay circuit 10. In FIG. 3, reference numerals 11, 12 and 13 represent high potential detection circuits (hereinafter called MAX) each of which is constructed as shown in FIG. 4A for outputting a higher potential signal between the two signals inputted thereto, whereas reference numerals 14, 15 and 16 represent low potential detection circuits (hereinafter called MIN) each of which is constructed as shown in FIG. 4B for outputting a lower potential signal between the two signals inputted thereto.
Three line video signals of the NTSC system in the vertical direction of a screen can be classified into three patterns including a flat pattern shown in FIG. 5A, a step pattern shown in FIG. 5B, and a pulse pattern shown in FIG. 5C. In FIGS. 5A to 5C, n represents a point (present) on the arbitrary line of a raster, (n-1) represents a point (past) on the just previously scanned line, and (n+1) represents a point (future) on the line scanned next. If a vertical correlation is present among Cc signals, for example, which are modulated on a subcarrier having a frequency f.sub.SC =(455/2) f.sub.H where f.sub.H is a horizontal scanning frequency, then a pulse pattern shown in FIG. 5C which alternately changes at each line is obtained. This pulse pattern cannot be obtained through a conventional comb filter using two lines.
The fundamental operation of the filter circuit shown in FIG. 3 will be described with reference to FIG. 6. MAX 11 outputs a higher potential signal between signals C and B, MAX 12 outputs a higher potential signal between signals B and A, and MIN 14 outputs a lower potential signal X(+) between the outputs from MAX 11 and MAX 12, the signal X(+) being represented by MIN (MAX (C, B), MAX (B, A)). Similarly, MIN 15 outputs a lower potential signal between signals C and B, MIN 16 outputs a lower potential signal between signals B and A, and MAX 13 outputs a higher potential signal X(-) between the outputs from MIN 15 and MIN 16, the signal X(-) being represented by MAX (MIN (C, B), MIN (A, C)). The signals X(+) and X(-) are added together at the adder 17 and divided by 1/2 at a 1/2 circuit 18 to obtain a signal Cc=(X(+)+X(-))/2. Since the signal Cc is obtained first in this circuit, the inverted present line signal is used for the above operation.
The output of the filter circuit 3 is therefore represented by a formula (B+MID (A, B, C,))/2, where MID (A, B, C) represents the second highest level signal among the three inputted signals A, B and C. Resultant Cc signals for the four patterns shown in FIGS. 7A to 7D take values as indicated at the right side column.
With the conventional circuit shown in FIG. 3, however, there is a problem that when a vertically striped image like multi bursts with repeated black and white as shown in FIG. 8A is displayed, there occurs cross colors at the upper and lower ends of the image, and shading of Yc signal, respectively as shown in FIG. 8B. In particular, if the vertically striped image is present below B as shown in FIG. 8A, the signals of the three line information at the upper ends become A=0, B=C=1 so that with signal B inverted for generating Cc signal, the resultant signals become A=0, B=-1, and C=1 as shown in FIG. 9B. Using the output formula of (B+MID (A, B, C))/2, the Cc signal becomes (B+A)/2=-1/2 so that the image is unnecessarily colored (cross colored). In addition, the Yc signal becomes 1-(1/2)=1/2 so that the amplitude thereof is halved, thus posing a problem of shading.
In order to solve the above problems, the present applicant filed a Japanese Patent Application entitled "Yc separation circuit" on Oct. 22, 1987. According to this circuit, obtained first is a mean value signal of past line information and future line information. If the mean value signal and a color signal with unnecessary signal components have the same sign, a lower level signal is selected, whereas if both signals have a different sign, a zero level signal is selected irrespective of the color signal level. Then, the color signal with unnecessary signal components is subtracted by the smaller level signal or the zero level signal to thus obtain a color signal with less unnecessary signal components.